1. Field of the Invention
This invention relates to an attachment structure for an output sprocket wheel attached to an output power shaft of a power unit internal combustion engine.
2. Description of Related Art
An example wherein a speed change gear is incorporated in an engine case of an internal combustion engine and a countershaft (driven gear wheel shaft) of the speed change gear is used as an output power shaft which has an output sprocket wheel attached to an end portion thereof (refer to, for example, Japanese Patent Laid-Open No. Hei 5-085460).
In the multistage speed change gear disclosed in Japanese Patent Laid-Open No. Hei 5-085460, an end portion of a countershaft (driven gear wheel shaft) has involute spline engagement teeth formed at a reduced diameter portion thereof which is reduced in diameter and is formed with a stepped portion. An output sprocket wheel is spline-fitted with the stepped portion in such a manner as to sandwich a collar member therebetween, and a bolt is tightened to an end face of the countershaft with a washer interposed therebetween.
In particular, the output sprocket wheel attachment structure disclosed in Japanese Patent Laid-Open No. Hei 5-085460 is structured such that the output sprocket wheel is positioned and fixed by the collar member between the stepped portion and the washer.
Accordingly, the output sprocket wheel cannot move in an axial direction at all and cannot absorb a force component which is caused by wrapping of a chain and waves in the axial direction. Therefore, the output sprocket wheel cannot transmit power stably.